Non-linear transport and optical phenomena in two-dimensional (2D) electron systems are at the core of modern research in solid-state physics [1]}, [2]}, [3]}, [4]}.
Of particular interest are the second-order effects comprising second harmonic generation (SHG) [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, [12]}, [13]}, [14]}, [15]} and generation of dc current by ac electric field of radiation [16]}, [17]}, [18]}, [19]}, [20]}, [21]}, [22]}, [23]}. Such effects in the leading electro-dipole electron-photon interaction occur in structures with broken space inversion symmetry and, therefore, have been established as a sensitive tool to probe structural inhomogeneity, crystalline symmetry, the staking and twist of 2D crystal flakes, etc. [6]}, [25]}, [26]}, [27]}, [28]}, [29]}.
